In reply to Team BMC:
I think it unlikely the adder is 'nesting'. Most snakes do lay eggs but adders are viviparous, giving birth in late summer, and it doesn't sound like a likely spot for a hibernacula or habitual basking spot either. If there were eggs it would be a grass snake, although they are usually laid somewhere well insulated. If an adder I think it's more likely the snake was just passing through. Great to hear there are snakes at Caley though!
Post edited at 10:57